In positron emission tomography (PET) imaging, a radiopharmaceutical agent is administered, via injection, inhalation and/or ingestion, to a patient. The physical and bio-molecular properties of the agent then concentrate at specific locations in the human body. The actual spatial distribution, intensity of the point and/or region of accumulation, as well as the kinetics of the process from administration and capture to eventual elimination, all have clinical significance. During this process, the positron emitter attached to the radiopharmaceutical agent emits positrons according to the physical properties of the isotope, such as half-life, branching ratio, etc. Each positron interacts with an electron of the object, is annihilated and produces two gamma rays at 511 keV, which travel at substantially 180 degrees apart. The two gamma rays then cause a scintillation event at a scintillation crystal of the PET detector, which detects the gamma rays thereby. By detecting these two gamma rays, and drawing a line between their locations or “line-of-response,” the likely location of the original annihilation is determined. While this process only identifies one line of possible interaction, accumulating a large number of these lines, and through a tomographic reconstruction process, the original distribution is estimated with useful accuracy. In addition to the location of the two scintillation events, if accurate timing—within few hundred picoseconds—is available, time-of-flight calculations are also made in order to add more information regarding the likely position of the annihilation event along the line. A specific characteristic of the isotope (for example, energy of the positron) contributes (via positron range and co-linearity of the two gamma rays) to the determination of the spatial resolution for a specific radiopharmaceutical agent.
The above process is repeated for a large number of annihilation events. While every case needs to be analyzed to determine how many scintillation events are required to support the desired imaging tasks, conventionally a typical 100 cm long, FDG (fluoro-deoxyglucose) study accumulates about 100 millions counts or events.
As shown in FIG. 1, detection of the leading edge of an output pulse from a photosensor 2000 is traditionally performed via a threshold discriminator circuit 2001 connected to a time-to-digital converter 2002. In addition, the analog output signal from the PMT can be sampled with a filter 2003 and a relatively slow (<200 MHz or Msample/sec) analog-to-digital converter (ADC) 2004, and the peak or integral of the event can be extracted from the sampled data. The peak or integral is used to calculate the energy and position of the event, usually by combining measurements from multiple photosensors.
However, using a threshold discriminator circuit requires a set of analog circuits to manipulate the PMT signal prior to input to the discriminator. Each analog circuit contributes to loss of fidelity of the signal. Additionally, due to the need for multiple threshold circuits, information contained in the shape of the leading edge of the signal may be lost and the accuracy of the leading edge measurement is reduced.
Using a high bandwidth ADC can provide much finer granularity of the magnitude of the output signal. However, in order to obtain the large amount of information included in the leading edge of the signal, a large number of samples must be obtained by the ADC. Obtaining a large number of samples leads to a large amount of data being produced by the ADC, which is difficult to store and manage.